DALLAS (CBSDFW.COM) – There has been a possible security breach at Target stores across the country. Federal authorities have stated that the personal information of some 40 million customers may have been compromised. And it comes at the busiest shopping time of the year.

CBS News has confirmed that the Secret Service is investigating the security breach that may involve stolen credit card and debit card information from potentially millions of Target customers.

Closely watched security industry blog Krebs on Security said that the breach started on Black Friday and ran until just last week. The report states that investigators think that thieves stole the data from credit card machines in almost all Target stores nationwide.

Investigators believe that the data was obtained via software installed on the machines that customers use to swipe magnetic strips on their cards when paying for merchandise at Target stores. It is not yet clear how the attackers were able to compromise point-of-sales terminals at so many Target locations.

Those strips on the backs of credit cards and debit cards are full of information. “Your address, your date of birth, your social security number,” explained consumer credit expert Paul Oster. “Once they access that, it’s very easy for them to open up other credit cards or gift cards.”

Information law expert William McGeveran added, “It sounds like the actual payment software was hacked with a piece of malware or some other kind of security breach in the actual credit card processing itself, and that’s not very common.”

Krebs on Security also reported that there is no indication that the theft affected shoppers on Target’s website.

Target released a statement early Thursday morning. It said, in part, “Target’s first priority is preserving the trust of our guests and we moved swiftly to address this issue. We regret any inconvenience this may cause.” The company also said that the issue has been fixed and that customers who suspect any unauthorized activity on their cards are encouraged to call them at 866-852-8680.

American Express stated that they are aware of the incident and have put fraud controls in place. Representatives for MasterCard and Visa declined to comment on the issue.

Most customers on Thursday morning said that they had heard about the security issue, but were still comfortable with shopping at a Target store. “We spend a lot of money here, so little worried about the breach of security, I guess you could say,” said Jessica Miller.

“Not overly concerned about it,” added Greg Brendel. “I figure if it gets some charges that aren’t on there, I’m sure they’ll take care of it.”

“I’ve had that happen before,” said Debbie Dunlap, “where a merchant, there’s been a breach of security and they automatically just changed by credit card when they found out. So, I’m not really worried. But I probably won’t go into Target right now and charge, I’ll probably use cash.”

“First of all, if you used a credit card at Target in the past few weeks, don’t panic,” advises Carrie Hurt, President and CEO of the Council of Better Business Bureaus. “You are not liable for any fraudulent charges on your account, and there are some simple things you can do to make sure your card was not used fraudulently.”

The local BBB has an additional warning. Jeannette Kopko, Vice President of Communications of the Dallas Better Business Bureau, says this initial breach likely will trigger another threat.

“Scammers love to follow up on an event like this and piggyback on it and get your attention and get something from you. They may use phishing e-mails where they claim to be from Target or from your bank.” She warns to be suspicious of unsolicited e-mail. “Beware of phishing e-mails and phone calls from businesses you don’t know; you don’t want to click on links or open attachments when you don’t know who the e-mail is from.”

For those who shopped at Target with a credit card, the BBB suggests you monitor your credit card statements carefully (go online; don’t wait for the paper statement) and report any fraudulent charges to your bank or credit card issuer immediately so the charge can be reversed and a new card issued. You should also keep receipts in case you need to prove which charges you authorized and which ones you did not. The BBB advises debit card shoppers follow the same steps, but keep in mind that debit cards do not have the same protections as credit cards and debit transactions withdraw funds directly from your bank account. Debit card users can contact their banks for more information, request a new card or put a security block on their account.